The present invention relates to a positioning method, a storage medium, a positioning device, and an electronic instrument.
The global positioning system (GPS) is widely known as a satellite positioning system, and is utilized for a car navigation system and the like. In the GPS, GPS satellite signals are respectively transmitted from a plurality of GPS satellites which orbit the earth, and a GPS receiver calculates (locates) its present position based on the received GPS satellite signals.
A GPS satellite signal affected by a multipath or the like may be included in the acquired GPS satellite signals. The term “multipath” refers to a phenomenon in which an indirect wave reflected or diffracted by a building or topography is superimposed on a direct wave from a GPS satellite so that the GPS receiver receives identical radio waves through multiple paths. Such a reception environment is referred to as a multipath environment. The present position of the GPS receiver may not be accurately calculated (located) when using a GPS satellite signal affected by a multipath. To deal with this problem, a method has been known which improves positioning accuracy utilizing an altitude table (urban altitude information) in a multipath area in which a number of buildings such as high buildings are present (e.g., the center of Tokyo). The altitude table is a data table in which a central urban area having a small difference in altitude is divided into a predetermined number of (i.e., nine) mesh areas and the altitude corresponding to each mesh area is specified (see JP-A-2006-177783, for example).
The GPS receiver generally calculates its position as follows. Specifically, the GPS receiver selects satellite sets (i.e., combinations of four or more GPS satellites) based on the acquired GPS satellite signals, and performs positioning calculations corresponding to the respective satellite sets to calculate candidate present positions. The GPS receiver evaluates the positioning accuracy of the candidate present positions calculated corresponding to the respective satellite sets based on an index such as a position dilution of precision (PDOP), and determines the candidate present position having the highest evaluation result to be the present positioning result.
However, the positioning accuracy of each satellite set does not differ to a large extent in a reception environment with high overall positioning accuracy (e.g., open-sky environment). On the other hand, GPS satellite signals are affected by a multipath to a different extent in a reception environment with poor positioning accuracy (e.g., multipath environment). As a result, the positioning accuracy of each satellite set may differ to a large extent. In this case, it is important to appropriately determine a satellite set with high positioning accuracy. According to related-art technology, the candidate present position of each satellite set is evaluated using an evaluation method similar to that used for a reception environment with high positioning accuracy (e.g., open-sky environment).